Conventionally, a compact power inductor has been known. The compact power inductor is used for realizing functions such as suppressing noise in a signal, rectification, smoothing signals, for example, in a power supply circuit for semiconductors, and a power circuit such of a DC-DC convertor, and the like. A wirewound inductor and a layered (multi-layer) inductor are known, as such a compact power inductor.
The compact power inductor needs to be compact, and to have a large inductance, a low resistance, and an excellent superimposed DC current characteristic, and so on. It is said that the superimposed DC current characteristic is excellent, if a magnetic saturation does not occur (magnetic permeability in a magnetic path does not become small), and accordingly, its inductance does not decrease, even when a relatively large DC current signal is applied to a coil in addition to an AC current signal (i.e., even when a large superimposed DC current is flowed in the coil).
As shown in FIG. 27 showing a cross sectional view of a conventional wirewound inductor 100, the conventional wirewound inductor 100 includes a core (magnetic core) 101 and a coil (conductive wire) 102. The coil 102 is helically wound around the core 101. When a current is flowed in the coil 102 of the wirewound inductor 100, the magnetic path is formed as shown by a dashed line in FIG. 27. The magnetic path passes through a space. That is, the wirewound inductor 100 has an open magnetic circuit configuration. Accordingly, since the magnetic flux density is hardly excessive and the magnetic saturation scarcely occurs, the superimposed DC current characteristic of the wirewound inductor 100 is therefore relatively good. However, in order to increase the inductance of the wirewound inductor 100, the fine conductive wire 102 needs to be wound around the core 101 with a large number of turns. This causes a problem that the resistance becomes large. Moreover, manufacturing processes for the wirewound inductor 100 are complicated, and there is a limit to downsizing the wirewound inductor 100.
To the contrary, a conventional layered inductor 110, as shown in FIG. 28 showing a perspective view of the inductor 110 and FIG. 29 showing a cross sectional view of the inductor 110, comprises a magnetic body 111, a coil 112 buried in the magnetic body 111, and a pair of electrode terminals 113. The coil 112 is formed of thin plate-like conductors 112a, each of which is formed to have a predetermined shape in each of layers, and conductors 112b in the via holes electrically connecting between the plate-like conductors 112a of the layers in a vertical direction. The pair of electrode terminals 113 are formed at both end portions of the magnetic body 111.
When a current is flowed in the coil 112 of the layered inductor 110, the magnetic path is formed as shown by a dashed line in FIG. 29. This magnetic path passes through the magnetic body 111 only. That is, the layered inductor 110 has a closed magnetic circuit configuration. Accordingly, since the layered inductor 110 has a high inductance even if a number of turns of the coil 112 is relatively small, the inductor 110 can decrease its resistance and be downsized. However, as schematically shown in FIG. 30, the magnetic flux density becomes extremely large in the neighborhood of the coil 112, when a current is flowed in the coil 112. Accordingly, the superimposed DC current characteristic of the layered inductor 110 is not good, because the magnetic saturation easily occurs.
FIG. 31 shows a cross sectional view of “a conventional layered inductor 120” coping with the problem described above. The layered inductor 120 comprises a first outer body 121 made of ceramic, a resin layer 122, an intermediate body 123, a resin layer 124, a second outer body 125, a core 126, and a helically wound coil conductor 127. The core 126 is formed at a central portion of the intermediate body 123 and of the second outer body 125. The coil conductor 127 is formed so as to surround the core 126. Each of the first outer body 121, the second outer body 125, and the core 126 is composed of a high magnetic permeability material. The intermediate body 123 is composed of a low magnetic permeability material. Accordingly, in the layered inductor 120, a part of the magnetic path is an open magnetic circuit as shown by a dashed line in FIG. 31. As a result, the magnetic flux density hardly becomes excessive, the magnetic saturation therefore scarcely occurs. Consequently, the layered inductor 120 which is compact and shows excellent superimposed DC current characteristic is provided (refer to, for example, a Patent Document 1).